As a comment... You might be able to schedule to the second but SMS delivery is never guaranteed. It can be delivered in a few seconds, or even hours. And are you sure the device itself has no clock skew?

In my experience the other variables you mention can be accounted for with a bit of practice. With my old Symbian Nokia I do it manually and know exactly how many seconds a text will take to my target. I assume the fact that I'm always sending to the same address makes it consistent and simple to calculate.

In my experience the other variables you mention can be accounted for with a bit of practice. With my old Symbian Nokia I do it manually and know exactly how many seconds a text will take to my target. I assume the fact that I'm always sending to the same address makes it consistent and simple to calculate.

That will sometimes work.

You'll never be able to account for the actual time taken from network A to network A, B, C or D. Time of day issues, location issues and load issues (Is there a concert nearby?) will all potentially affect delivery time for SMS.